The present invention relates to a novel heat-sensitive color transfer recording media and a printing process using the recording media. More particularly, it relates to a heat-sensitive color transfer recording media suitable for producing color images in electroresistive recording system.
In recent years, electoresistive recording system as well as thermal recording system using thermal printer or thermal facsimile have been rapidly developed because of their advantages such as low noise, maintenance free and low cost. The principle of the electroresistive recording system is shown in FIG. 1. A recording media 100 used in the electroresistive recording system is usually composed of an electroresistive layer 101, an electrically conductive layer 102 and a heat-sensitive ink layer 103. A printing head 110 used in the electroresistive recording system is composed of a plurality of needle-like electrodes 111 held electrically insulated from each other in a fixed relationship by insulation. The printing head 110 is brought in contact to the electroresistive layer 101 of the media 100. Thus, an electric circuit can be established through any one of the electrodes 111 activated selectively, the electroresistive layer 101 and a return electrode 112 which is grounded. In that case, an electric current applied to the electroresistive layer 101 from the electrode 111 flows to an electrically conductive layer 102 through the electroresistive layer 101 due to the suitable electric resistance of the electroresistive layer 101, by which Joule heat is generated. The Joule heat causes the melting of the heat-sensitive ink layer 103 and the melted ink 103a is transferred onto a receiving medium 105, such as plain paper.
Known electroresistive recording system is of the type producing a duplicate of single color such as black, blue or red, and there is a strong desire for electroresistive recording system capable of producing multi-color images.
In color recording system technologies, there are known an impact recording system as seen in a conventional typewriter in which a cloth ribbon, e.g. a ribbon coated with inks of two colors, is employed; and an ink-jet recording system in which inks are jetted by employing two or more ink-jet heads. The former has the disadvantages of generating much noise and slow recording speed. Also, the latter has the disadvantage that problems such as clogging of the nozzle for jetting out ink tend to occur since the ink is jetted through the nozzles having a very narrow opening, and since the amount of ink jetted must be controlled, the apparatus itself is very complicated and expensive and the operation thereof is also complicated.
Recently, to make available a color printing system which overcomes the aforementioned problems, there has been proposed a color recording system utilizing the principle of color television and color phototelegraphy. FIG. 2 is a diagrammatic view showing such a color recording system. A color original 1 is subjected to color separation by filters 2a, 2b and 2c. The respective color-separated images are then read by photoelectric tubes 3a, 3b and 3c and are converted into yellow signal Y, cyan signal C and magenta signal M. The signals are transmitted from a transmitter 4 to a receiver 5 at which the signal separation is conducted again. The signals are transmitted to a printer 6 to reproduce a color image which closely resembles the original, on a copy sheet 7. Recording machines applicable to such a system have been proposed and developed. The application of a printer for electroresistive recording system to the printer 6 is advantageous from view-points of low cost, ease of operation, low noise and high printing speed.
Conventionally known color image forming processes by the thermal color recording system include a process employed in a thermal color printer which makes it possible to obtain multi-color images in a single treatment as disclosed in Japanese Patent Unexamined Publication (Tokkyo Kokai) No. 156647/1979. This process uses a recording media comprising a foundation and heat-sentive ink spots provided thereon wherein 4-color ink spots of yellow, magenta, cyan and black colors are arranged in rhomboid cells and the rhomboid cells are disposed side by side in parallel rows which extend across the foundation. The thermal head used in this process is composed of a double row of heating elements arranged in rhomboid cells corresponding to the rhomboid cells of ink spots. Minute spots of different colors are transferred from the recording media onto a copy sheet by heating with the thermal head so that different color spots are not superimposed with each other to form a color image. The publication also discloses another process wherein there are used a recording media which has parallel stripes of heat-sensitive inks of the above four colors on a foundation, the stripes extending perpendicular to the travel direction of the recording media and being disposed in a repeating series of four colors (each stripe has a very narrow width of about 0.23 mm.) and a thermal head which has heating elements(dots) arranged in a single row, whereby each color is transferred line-to-line (width of each line: about 0.23 mm. and space between the lines: 0.025 mm.) on a copy sheet.
However, the above processes have problems that the color resolution is not satisfactory and also positions between the recording media and the copy sheet cannot easily be adjusted and, therefore, it is very difficult to obtain clear color images.
In particular, since each of the heat-sensitive ink spots or stripes disclosed in the above publication contains sublimiable dyes which are sublimated by the heating with a thermal head upon printing and deposited on a copy sheet, the use of the heat-sensitive recording media proposed therein accompanies fatal disadvantages that heat control of the thermal head is very difficult during printing and also the dyes of each color tend to be sublimated and mixed with each other during the storage of the media, whereby clear images cannot be obtained.
It is an object of the present invention to provide a heat-sensitive color transfer recording media which can be used in an electroresistive recording system to give clear multi-color images of high resolution at low cost.
Another object of the present invention is to provide a heat-sensitive color transfer recording media which makes it possible to use a low cost, small-size color printer for electroresistive recording system.
A further object of the present invention is to provide a heat-sensitive color transfer recording media suitable for high-speed color recording.
A still further object of the present invention is to provide a process for producing color images on a receiving medium such as plain paper according to an electroresistive recording system.
These and other objects of the present invention will become apparent from the description hereinafter.